1. Field of Invention
The present invention relates in general to heat exchanger tubes used in the construction of heat exchangers, air conditioning and refrigeration systems having heat exchangers using heat exchanger tubes, and to methods of manufacturing heat exchangers. More specifically, the invention relates to the arrangement of the inner surface of heat exchanger tubes.
2. Description of Related Art
Typically, air conditioners and refrigerators have a refrigerating circuit including a compressor, an expansion valve and heat exchangers. Refrigerant is circulated through the refrigerating circuit by the compressor. The heat exchangers facilitate the exchange of heat between an outside fluid (usually air) and refrigerant flowing therethrough. It is desirable for the heat exchanger to achieve a high heat transfer coefficient so that the refrigerating circuit will operate with high efficiency. In other words, it is desirable to achieve high air conditioning capacity or refrigerating capacity with low energy consumption.
FIG. 12 (Prior Art) shows the general arrangement of a known heat exchanger. A plurality of heat exchanger tubes 103 pass through faceplate fins 101 made of a high thermal conductivity metal material such as an aluminum. Heat exchanger tubes 103 are also made of a high heat conductivity material that is easy to process, such as a copper. The refrigerant flowing in heat exchanger tube 103, indicated by the arrow in FIG. 12, exchanges heat with a fluid flowing the space between adjacent plate fins 101.
It is known to use chlorofluorocarbon refrigerant CFC12 (called "R12") and hydrochlorofluorocarbon refrigerant HCFC22) (called "R22") as the refrigerants for air conditioning and refrigeration systems. Generally, R12 is used in refrigerators and R22 is used in air conditioners. These refrigerants are so-called "single refrigerants" because they have a single boiling point. This property makes them stable in changing between liquid state and gaseous state. It is therefore easy to design systems using these refrigerants.
It has now become known that R12 is an environmental hazard. This refrigerant is chemically extremely stable in the atmosphere (doesn't break down) and tends to damage the earth's ozone layer. Even small amounts of R12 refrigerants which leak into the atmosphere, tend to accumulate in the ozone layer and damage it. Accordingly, R12 refrigerant has been designated as a specific Freon subject to regulations limiting its use. R22 refrigerant, on the other hand, decomposes in the atmosphere. It has far less potential for damaging the earth's ozone layer. However it does have some harmful effects. Therefore, it has been decided to limit the use of R22 as well.
Recently, HFC (hydrofluorocarbon) refrigerants which do not damage the ozone layer have been developed as substitutes for those which have been specifically designated to be harmful. Some of the recently developed Freons are:
1,1,1,2-tetrafluoroethane (R134a), PA1 pentafluoroethane (R125), PA1 1,1,2,2-tetrafluoroethane (R134), PA1 1,1,2-trifluoroethane (R143), PA1 1,1,1 trifluoroethane (R143a) 1, PA1 1-difluoroethane (R152a) PA1 Inonofluoroethane (R161), etc. PA1 stacking a plurality of plate fins, each of the plate fins having a hole, with a predetermined space therebetween; PA1 inserting through the holes of the plate fins a heat exchanger tube having a tube wall configured so as to have an outer surface and an inner surface having a convex portion having a tip which has a predetermined area, and a plurality of inner fins each having a tip, the area of the tip of the convex portion being larger than that of the tip of the inner fins and the outer diameter of the heat exchanger tube being smaller than the diameter of the hole of plate fin; and PA1 expanding the heat exchanger tubes to engage the plate fins. The tubes themselves can have the various characteristics as described above with respect to the various tube embodiments. PA1 inserting into the tube a first expanding element having a diameter larger than that of the inside diameter of heat exchanger tube; and PA1 inserting into the tube a second expanding element having a diameter larger than that of the first expanding element. PA1 inserting into the heat exchanger tube a sleeve having a slit therein so that the diameter of the sleeve can expand; and PA1 inserting into the heat exchanger tube an expanding element having a diameter that is larger than the inside diameter of the sleeve.
The above-listed HFC refrigerants are single refrigerants. However, each of the HFC refrigerants has some disadvantages. For example, difluoromethane (R32) has a higher discharge pressure (and temperature) than that of R22 refrigerant. Unfortunately, a single HFC refrigerant having properties close to those of R12 or R22 has not yet been developed. Therefore, efforts have been made to develop a mixed HFC refrigerant, a mixture of two or more types of HFC single refrigerants that will work well in a refrigerating circuit.
Among the mixed refrigerants, R32/R125/R134a ("R407C") has properties somewhat similar to those of R22. However, all mixed refrigerants, including R407C, present a system design problem. Mixed refrigerants have multiple boiling points because each constituent refrigerant has its own boiling point that may be different from the boiling points of other constituents. Mixed refrigerants are therefore referred to as "zeotropic" refrigerants. When a zeotropic refrigerant evaporates in a heat exchanger, the refrigerant component having the lowest boiling point, for example in R407C, R32 has the lowest boiling point, evaporates first. At the same time, only a small amount of another constituent refrigerant in the mixture evaporates. Accordingly, when a heat exchanger is used as an evaporator, the high boiling point refrigerant constituent tends to remain in the heat exchanger, while the lower boiling point constituent evaporates. The constituent remaining in the heat exchanger tends to interfere with heat transfer of the heat exchanger, thus, the heat transfer coefficient is decreased. A problem also exists when the heat exchanger is used as a condenser. The low boiling point constituent tends to remain in the heat exchanger. Consequently, when a zeotropic refrigerant is used in refrigerating circuit, the heat transfer coefficient is decreased.
FIGS. 13 and 14 are graphs showing the heat transfer coefficients of a zeotropic refrigerant and single refrigerant. FIG. 13 depicts the heat transfer coefficients at the time of condensation and FIG. 14 shows the heat transfer coefficients at the time of evaporation. In both figures, the upper line is drawn through data points representing a single refrigerant while the lower line is drawn through data points representing a zeotropic refrigerant. As shown in the graphs, the zeotropic refrigerant has a lower heat transfer coefficient than that of a single refrigerant. Thus, refrigerating circuits using zeotropic refrigerant have a lower efficiency than those using a single refrigerant.
In an effort to overcome this problem it has been proposed to try to improve heat exchangers by improving their heat exchange tubes. The tubes have been provided with inner fins to improve their heat transfer coefficient. In this regard see U.S. Pat. No. 4,658,892--Shinohara et al. The inner surface of the tube disclosed therein has a number of spiral grooves defined by depth, shape and helix angle. The subject matter of U.S. Pat. No. 4,658,892 is hereby incorporated by reference as if fully set forth herein. Providing inner fins in the heat exchange tube increases its inner surface area touching the refrigerant which tends to increase its heat transfer coefficient.
FIG. 15 is a graph showing the improvement of heat transfer coefficient of a heat exchanger that can be achieved by using heat exchange tubes having inner fins. If the density of inner fins increases, the heat transfer coefficient of the heat exchanger is improved. However, there are practical problems associated with the manufacture of the heat exchange tubes and assembly of the tubes into the heat exchanger.
It is necessary to tightly connect the heat exchange tubes with the plate fins of the heat exchanger to increase its heat transfer coefficient. Generally, in manufacturing a heat exchanger, a plurality fins having holes are arranged parallel to each other, then the heat exchanger tubes are passed through the holes of the plate fins. After that, the heat exchanger tubes are expanded in a radial direction by inserting an expanding jig from one end of the heat exchanger tube. However, in a heat exchanger using inner fin heat exchanger tube, when the expanding jig is inserted, the inner fins on the surface of the heat exchanger tube are pushed in the radial direction by the expanding jig. As the result, the inner fins are mashed and deformed. This deformation decreases the inner surface area and reduces heat transfer efficiency. Also, the deformation may interfere with the flow of refrigerant in the tube. As flow resistance increases, refrigerant flow rate decreases. Refrigerant flow rate is one of the more important factors in determining refrigerating capacity and operating efficiency of the refrigerating circuit. When the flowing resistance increases, that is to say flowing speed of the refrigerant is decreased, the refrigerating capacity and the operation efficiency of the refrigerating circuit is decreased.